https://support.google.com/mail/bin/answer.py?hl=en&answer=13287
Gmail POP and SMTP server settings.
http://mandrill.com/
Mandrill, deliver incoming email to a webhook, and process outgoing email. Can do message manipulation also. 12k emails/month free, from MailChimp.
http://lightningbase.com/
Hosted WordPress service which includes an affiliate program that pays 20% for the lifetime of the account, plans start from $10/month.
http://www.controlledmail.com/
Controlled Mail is an outbound mail service (SMTP provider) with a very strong focus on genuine mail delivery. They accept no bulk mailings whatsoever and support SPF and DKIM by default. They also prevent cross client forgery.
http://phpmailer.sourceforge.net/
An open source mailer class for PHP, supports local sendmail, smtp, the full monty.